Treatment of magnesium



l Patented Sept. 12, 1944 TREATMENT OF MAGNESIUM Krcigh B. Ayers, Akron, Ohio, assignor to Wingfoot Corporation, Akron, Ohio, a corporation of Delaware No Drawing. Application November 12, 1941, Serial No. 418,859

Claims.

This invention relates to a method of treating magnesium and magnesium alloys and, more particularly, to a method of cleaning articles composed thereof to remove therefrom various undesirable impurities and contaminations without substantially attacking the magnesium itself.

In recent years, magnesium alloys have come into increasingly widespread use for various purposes, for example being employed for the production of wheels, brakes, etc. Articles prepared therefrom frequently contain impurities arising from various sources which must be removed from the surface of the article during its processing or before it can be put to use. One of the commonly encountered impurities is magnesium hydroxide formed by corrosion of the magnesium in the presence of water or water vapor. Another impurity is elemental sulfur remaining from the'sulfur used to prevent ignition of the magnesium during hot pouring in the foundry. Still others are borax and sodium silicofiuoride from fluxes employed. In removing these and other impurities, it is desirable to avoid attacking the magnesium, this consideration becoming especially important when the cleaning operation is applied to machined surfaces and it is necessary to maintain machine tolerances. Many cleaning methods necessarily result in more or less attack on the magnesium.

It is an object of the present invention to clean articles comprising magnesium in such a manner as to remove from the surface thereof a variety of the commonly encountered contaminations without, however, causing any substantial attack on the magnesium itself.

Chromic acid, 1. e., an aqueous solution of chromium trioxide, is an effective cleaning agent. It removes surface corrosion (magnesium hydroxide) by neutralization; it removes sulfur by'oxidation; it dissolves borax; it removes sodium silicofiuoride by oxidation to form volatile silicon tetrafluoride. However, chromic acid also attacks the metal, magnifying surface pits and pores and destroying machine tolerances.

According to the present invention, th attack of the magnesium or the magnesium alloy is substantially completely prevented by adding to the aqueous chromic acid a small amount of chromium trifluoride, which acts as a specific negative catalyst or inhibitor.

The concentration of chromic acid used may be varied over a wide range, good results being obtained with solutions containing about -30% of chromic acid (figured as CrOs) and a solution containing about working very well. The chromium trifluoride is relatively expensive and it is desirable to avoid using more than the required amount, but only very small or catalytic quantities are necessary so that the process works out nicely in this respect. Ordinarily a concentration of not more than 0.5 to 1% of chromium trifluoride is required and concentrations in the neighborhood of 0.1% have been found to function very well. The temperature of the solution should be at least about F. in order to remove all of the undesirable contaminations. The upper temperature limit is ordinarily set in practice by the boiling point of the solution. It has been found that temperatures at or near the boiling point i. e., about 212-215 F., are very satisfactory.

The chromic acid and chromium trifluoride are preferably preformed and, although no particular methods for forming them are required, it is necessary to give due consideration to the methods of preparation and the character of the materials since certain impurities tend to interfere with the cleaning process to a more or less serious extent. For instance, nitrates, sulfates or chlorides tend to promote attack of the metal and the materials should preferably be as free of these impurities as is possible.

While the invention has been described with respect to certain preferred embodiments, it will be apparent that numerous variations may be made therein within the general knowledge and ability of those skilled in the art without departing from the spirit or scope of the invention.

I claim:

1. A method of cleaning a material selected from the group consisting of magnesium and magnesium alloys which comprises treating it with an aqueous solution of chromic acid and chromium trlfiuoride.

2. A method of cleaning a material selected from the group consisting of magnesium and magnesium alloys which comprises treating it with an aqueous solution of chromic acid and chromium trifiuoride at a temperature of at least about 150 F.

3. A method of cleaning a material selected from the group consisting of magnesium and magnesium alloys which comprises treating it with an aqueous solution containing about 15- 30% of chromic acid and a catalytic quantity of chromium trifiuoride at a temperature between about 150 F. and the boiling point of the solution.

4. A method of cleaning a; material selected from the group consisting of magnesium and magnesium alloys which comprises treating it with an aqueous solution containing about 20% of chromic acid and a catalytic quantity of chromium trlfiuoride at a temperature of about 212- 215 F.

5. A method of cleaning a material selected from the group consisting of magnesium and magnesium alloys which comprises treating it with an aqueous solution of chromic acid containing a catalytic quantity of chromium triiiuoride at about the boiling point of the solution.

KREIGH B. AYERS. 

